<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="base.css">
</head>
<style>
    .box{
        position: absolute;
        top: 0;
        height: 0;
        width: 100%;
        height: 100%;
        background-color: aqua;
    }
</style>
<body>
    <div class="box"></div>
</body>
</html>